Telecommunications access systems provide voice, data, and multimedia transport and control between the central office (CO) of the telecommunications service provider and the subscriber (customer) premises. Prior to the mid-1970s, a subscriber was connected by phone lines (e.g., voice frequency (VF) pairs) directly to Class 5 switching equipment located in the central office of the telephone company. In the late 1970s, digital loop carrier (DLC) equipment was added to the telecommunications access architecture. The DLC equipment provided an analog phone interface, voice CODEC functions, digital data multiplexing, a transmission interface, and control and alarm functions remotely from the central office to cabinets located within business and residential locations for approximately 100 to 2000 phone line interfaces. This distributed access architecture greatly reduced line lengths to the subscriber and resulted in significant savings in both wire installation and maintenance. The reduced line lengths also improved communication performance on the line provided to the subscriber.
By the late 1980s, the limitations of data modem connections over voice frequency (VF) pairs were becoming obvious to both subscribers and telecommunications service providers. ISDN (Integrated Services Digital Network) was introduced to provide universal 128 kbps service in the access network. The subscriber interface is based on 64 kbps digitization of the VF pair for digital multiplexing into high speed digital transmission streams (e.g., T1/T3 lines in North America, E1/E3 lines in Europe). ISDN was a logical extension of the digital network that had evolved throughout the 1980s. The rollout of ISDN in Europe was highly successful. However, the rollout in the United States was not successful, due in part to artificially high tariff costs which greatly inhibited the acceptance of ISDN.
More recently, the explosion of the Internet and deregulation of the telecommunications industry have brought about a broadband revolution characterized by greatly increased demands for both voice and data services and greatly reduced costs due to technological innovation and intense competition in the telecommunications marketplace. To meet these demands, high speed DSL (digital subscriber line) modems and cable modems have been developed and deployed. The DLC architecture was extended to provide remote distributed deployment at the neighborhood cabinet level using DSL access multiplexer (DSLAM) equipment. The increased data rates provided to the subscriber resulted in the upgrading of DLC/DSLAM transmission interfaces from T1/E3 interfaces (1.5/2.0 Mbps) to high-speed DS3 and OC3 interfaces. In a similar fashion, the entire telecommunications network backbone has undergone and is undergoing continuous upgrading to wideband optical transmission and switching equipment.
Similarly, wireless access systems have been developed and deployed to provide broadband access to both commercial and residential subscriber premises. Fixed wireless broadband systems use a group of transceiver base stations to cover a region in the same manner as the base stations of a cellular phone system. The base stations of a fixed wireless broadband system transmit forward channel (i.e., downstream) signals in directed beams to fixed location antennas attached to the residences or offices of subscribers. The base stations also receive reverse channel (i.e., upstream) signals transmitted by broadband access equipment at the subscriber premises.
Initially, the market for wireless access systems was driven by rural radiotelephony deployed solely to meet the universal service requirements imposed by government (i.e., the local telephone company is required to serve all subscribers regardless of the cost to install service). The cost of providing a wired connection to a small percentage of rural subscribers was high enough to justify the development and expense of small-capacity wireless local loop (WLL) systems.
Deregulation of the local telephone market in the United States (e.g., Telecommunications Act of 1996) and in other countries shifted the focus of fixed wireless access (FWA) system deployment from rural access to competitive local access in more urbanized areas. In addition, the age and inaccessibility of much of the older wired telephone infrastructure make FWA systems a cost-effective alternative to installing new, wired infrastructure. Also, it is economically more feasible to install FWA systems in developing countries where market penetration is limited (i.e., the number and density of users who can afford to pay for services is limited to a small percentage of the population) and where wired infrastructure cannot be installed profitably. In either case, broad acceptance of FWA systems requires that the voice quality and data integrity of FWA systems meet or exceed the performance of wired infrastructure.
Wireless access systems must address a number of unique operational and technical issues including:
1) Relatively high bit error rates (BER) compared to wire line or optical systems; and
2) Transparent operation with network protocols and protocol time constraints for the following protocols:                a) ATM;        b) Class 5 switch interfaces (domestic GR-303 and international V5.2);        c) TCP/IP with quality-of-service QoS for voice over IP (VOIP) (i.e., RTP) and other H.323 media services; and        d) Distribution of synchronization of network time out to the subscribers;        
3) Increased use of voice, video and/or media compression and concentration of active traffic over the air interface to conserve bandwidth;
4) Switching and routing within the access system to distribute signals from the central office to multiple remote cell sites containing multiple cell sectors and one or more frequencies of operation per sector; and
5) Remote support and debugging of the subscriber equipment, including remote software upgrading and provisioning.
Unlike wireline systems (including optical) that operate at bit error rates (BER) of 10−11, wireless access systems have time varying channels that typically provide bit error rates of 10−3 to 10−6. The wireless physical (PHY) layer interface and the media access control (MAC) layer interface must provide modulation, error correction and ARQ protocol that can detect and, where required, correct or retransmit corrupted data so that the interfaces at the network and at the subscriber site operate at approximately wire line bit error rates.
RF propagation channels (i.e., links) between a subscriber transceiver and a base station transceiver may vary due to a number of link factors. The two most important factors are propagation loss and channel distortion. Depending of the type of channel, the propagation loss between a transmitter and a receiver increases beyond the best case line-of-sight (LOS) R2 propagation losses to include distortion caused by multipath, shadowing, and diffraction.
Propagation paths may be characterized as line of sight (LOS), non-line of sight (non-LOS) with knife-edge diffraction (non-shadowed), and non-LOS with shadowing or obstruction. In a LOS propagation path, there is no obstruction within the Fresnel zone. The Fresnel zone is the area around the line of sight that radio waves spread into after the radio waves leave the antenna. As a rule of thumb, if at least 55% of the first Fresnel zone is clear, then there is no significant distortion. In a LOS propagation path, multipath may still be present. The path loss of a LOS propagation path is approximately R2, where R is the path length.
In a non-LOS propagation path with knife-edge diffraction, a structure or terrain (e.g., hill) blocks the direct line of sight, but only partially blocks (i.e., non-shadowed) the Fresnel zone of the direct path. In this situation, RF waves diffracted around the obstruction (i.e., knife-edge diffraction) still reach the subscriber. In a non-LOS propagation path with knife-edge diffraction, the length of the propagation path is increased (excess path length), which increases the path loss beyond R2, thereby including an additional diffraction loss component.
In a non-LOS propagation path with shadowing or obstruction, the direct line of sight and the associated Fresnel zone are blocked (shadowed) by a structure or terrain. In this situation, the path loss is a combination of normal path loss and penetration losses through the obstruction. Establishment of a link is still possible where the attenuation of the obstruction still allows enough residual signal power to be received by the subscriber equipment. However, prediction of the link characteristics under these conditions is difficult.
For frequencies below 11 gigahertz (GHz), the primary source of channel distortion is multipath reflection caused by reflection of the transmitted signal off objects (e.g., buildings, terrain) in the area. Multipath reflection causes the primary (direct path) signal and a number of delayed path signals to arrive at the receiver at different times. One way of describing the extent of multipath is to define the delay spread of the channel auto-correlation delay profile. The delay spread is defined as the difference in time between the arrival of the first signal (H1 direct path) and the arrival of the last detectable signal (H3 longest multipath) that is X dB below the power of the first signal.
Two important papers provide detailed studies of the delay spread in 2 GHZ and 2.5 GHZ channels across a number of different line-of-sight and non-line-of-sight channels. J. W. Porter and J. A. Thweat provided a study of multi-point microwave distribution system (MMDS) frequency propagation in a suburban environment (“Microwave Propagation Characteristics in the MMDS Frequency Band,” Proceedings of the International Conference on Communications, New Orleans, June 2000). This study noted that a combination of directional transmit and receive antennas provided for root-mean-square (RMS) delay spread of less that on microsecond (1 usec.) in 90% of the link cases. The study also reported that lower antenna heights resulted in lower delay spread but also greater propagation loss due to non-line-of-sight conditions. A summary of the test results is provided below in TABLE 1.
TABLE 1RMS DelayRMS DelayRMS DelaySignalAntennaSpread Min.Spread Max.Spread MeanPathType(usec.)(usec.)(usec.)LOSDirect.0.020.040.02LOSOmni0.022.390.13Non-LOSDirect.0.025.260.14Non-LOSOmni0.027.060.37
A study by V. Erceg, D. G. Michelson and others provided a similar study at 2 GHZ (“A Model for Multipath Delay Profile for Fixed Wireless Channels,” IEEE JSAC, Volume 17, No. 3 March 1999, pp. 399–410). In this study, delay spread (full time span, not RMS delay spread) of up to one microsecond (usec.) was detected for both omni and directional antennas.
Propagation loss effects the energy level of the signal and ultimately the modulation complexity that can be supported. Multipath and the resulting delay spread can result in distortions that make the signal impossible to demodulate regardless of received energy level, unless some correction technique to combat multipath is implemented. A number of multipath correction techniques are known, including 1) signal processing to perform channel equalization (inverse filtering), 2) directional antennas (limit sources of multipath), and 3) spatial diversity receivers (demodulation and coherent combination of one or more antenna-receiver sources).
The choice of modulation and associated signal processing (i.e., equalization techniques) impacts the complexity of both the digital baseband modem and the linearity of the RF transceiver. Common modulation schemes have been summarized by Falconer and Ariyavistakul in a submission to the IEEE 802.16.3 working group (D. Falconer and S. Ariyavistakuo, “Modulation and Equalization Criteria for 2 to 11 GHZ Fixed Broadband Wireless Systems,” IEEE 802.16c-00/Sep. 13, 2000). These modulation schemes include: 1) orthogonal frequency division multiplexing (OFDM) based on a Fast Fourier Transform-Inverse Fast Fourier Transform (FFT/IFFT) implementation; 2) single carrier (SC) modulation with time domain adaptive equalization; and 3) single carrier modulation with frequency domain adaptive equalization.
There is a great deal of emphasis on OFDM modulation formats as opposed to single carrier modems that implement equalization. The support for OFDM modulation is based on the computational complexity of the equalizer being a linear function of delay spread in single carrier modems, while the computational complexity of the equalizer in an OFDM system is a log function of the delay spread using efficient FFT implementations. However, this benefit in computational complexity is gained at the cost of increased RF linearity, increased frequency sensitivity, and granularity problems in the OFDM systems.
Therefore, there is a need in the art for improved broadband wireless access equipment that is less expensive and more reliable than existing broadband wireless access equipment. In particular, there is a need in the art for base station receivers having very low bit error rates for use in broadband wireless access equipment. More particularly, there is a need for base station receivers having the computational complexity of OFDM systems without suffering the increased RF linearity, increased frequency sensitivity, and granularity problems associated with OFDM systems.